Friday, 10 January 2014

The geomagnetic storm that wasn’t.... by Dr Gemma Kelly

You may have been aware in the last few days of a lot of media excitement about a chance to see the Northern lights here in the UK. So, last night many people in the country (space weather experts included) were glued to computer screens, and in some cases the night sky itself, in anticipation of the aurora arriving....but then it never materialised!So why all the fuss? Well, let’s start at the beginning...

An image of the Sun from the Solar dynamicobservatory during the X-class flare (the brightarea near the centre) on 7th Jan.

On 7th January at around 18.30 there was an X1-class (moderately strong) solar flare close to the centre of the Sun. However, a flare by itself is not enough to cause an aurora here on Earth. Associated with this flare was a Coronal Mass Ejection (CME), which is a massive burst of charged gas and jumbled up solar magnetic field. In this case the CME appeared to be directed almost straight at the Earth and initial estimates of the speed it was travelling were high (around 2000 km per second). Given all that information most forecasters (including us here at the BGS) were in agreement that this CME was very likely to hit the Earth’s magnetic field at some point on the 9th January. When a CME arrives at Earth we expect to see a shock signature observed in the ACE satellite data. This means we see a sharp jump in the velocity and magnetic field of the solar wind – which is a continuous stream of charged particles released from the Sun. Sure enough, just after 19.00 on the 9th the shock arrived, so the CME had arrived – time to get excited (and tweeting). However, to get a geomagnetic storm, and therefore the Northern lights there are a few more complications....Firstly, the CME had arrived several hours later than expected suggesting it was a bit slower than first predicted - in general the faster the CME the bigger the resulting geomagnetic storm. Secondly, the shock was also quite small which might mean that we only received a glancing blow from the CME, and most of it missed us. Thirdly, and most importantly, to get a geomagnetic storm a CME really needs to cause the interplanetary magnetic field (the field trapped in the solar wind – or IMF) to turn southwards, which allows much more energy into the Earth’s magnetic field. Following last night’s CME arrival the IMF stayed stubbornly northwards, therefore, the geomagnetic storm never really got going.In short, space weather forecasting is really hard! We are continuously improving the way we forecast and model space weather, but until we can get more information about the CMEs before they reach us there will always be a lot of uncertainty.To get daily space weather forecasts follow @BGSspaceWeather and for those all important aurora alerts follow @BGSauroraAlert. Gemma @GeomagGem

Extra bitsFlares are caused by the explosive reorganisation of magnetic fields in the Sun’s atmosphere, and are classified according to how powerful they are as A (the least powerful), B, C, M or X (the biggest). X-class flares are the most powerful, ranging from X1 up to at least X28, where an X10 is 10 times more powerful than an X1. ‘Space weather’ is a term to describe the conditions in the space between the Sun and the Earth. Changes in space weather are almost exclusively driven by events on the surface of the Sun, and the Sun’s atmosphere.

To find out more on the science of geomagnetism go to our website here.